1. Field of the Invention
This invention relates to erosion and abrasion resistant overlays on the steel surfaces of earth boring bits.
2. Description of the Related Art
SOLIDIFICATION HARDMETALS
Hardmetal inlays or overlays are employed in rock drilling bits as wear, erosion, and deformation resistant cutting edges and faying surfaces.
The strongest commonly employed hardmetals used in rock drilling bits are made by weld application of sintered tungsten carbide based tube metals or composite rods using iron alloy matrix systems. Heat input during weld deposition of such overlays is critical. Practical control limitations normally result in matrix variation due to alloying effects arising from melt incorporation of sintered carbide hard phase constituents as well as substrate material. Partial melting of cemented carbide constituents results in "blurring" of the hard phase boundaries and the incorporation of cobalt and WC particles into the matrix. Process control is typically challenged to maintain "primary" hardmetal microstructural characteristics such as constituency and volume fraction relationships of hard phases. Secondary characteristics such as matrix microstructure are derivative and cannot be readily regulated.
These overlays typically comprise composite structures of hard particles in a tough metal matrix. The hard particles may be a metal carbide, such as either monocrystalline WC or the cast WC/W.sub.2 C eutectic, or may themselves comprise a finer cemented carbide composite material. Often, a combination of hard particle types is incorporated in the materials design, and particle size distribution is controlled to attain desired performance under rock drilling conditions, such as disclosed in U.S. Pat. No. 3,800,891, No. 4,726,432 and No. 4,836,307.
The matrix of these hardmetal overlays may be iron, nickel, cobalt, or copper based, but whether formed by weld deposition, brazing, thermal spraying, or infiltration, the matrix microstructure is necessarily a solidification product. During fabrication, the hard phase(s) remain substantially solid, but the matrix phase(s) grow from a melt during cooling and thus are limited by thermodynamic, kinetic, and heat transport constraints to narrow ranges of morphology, constituency and crystal structure.
Welded composite hard metals encounter several limitations when large areal coverage is needed such as in continuous overlays of bit cutting faces as shown in FIGS. 1 and 2. Foremost of these is the high cost of application. Also, compatibility issues provide physical limits arising from property differentials between substrate materials and overlays, and fabrication logistics become limiting due to thermal stability issues with substrate or cutting elements. These factors have limited welded composite rod hardfacing overlays to crest and flank locations of tooth type roller cone bit cutting structures, and have precluded their use in interference fitted (insert type) roller cone bit cutting structures.
Welded overlays have been incorporated for large areal protection of faces and gage surfaces of drag type polycrystalline diamond composite (PDC) bits. However, necessary compromises in coverage, constituency, and application method have rendered the performance/cost relationship marginal for many PDC products.
Welded hardmetal overlays are commonly used for protection of lug "shirttail" locations of both tooth and insert of roller cone bits, although coverage is necessarily selective, due to cost and the tendency to crack which increases with areal coverage.
Due to the aforementioned limitations, practice in both insert type roller cone and PDC drag bits has gravitated to thermal spray carbide composite coatings for erosion and abrasion protection of large areas. Various thermally sprayed coatings for drill bits are disclosed in U.S. Pat. Nos. 4,396,077; 5,279,374; 5,348,770; and 5,535,838. These coatings are typically too thin, too fine grained, and too poorly bonded to survive long in severe drilling service. In addition, consistency of thermal spray coatings is notoriously variable due to process control sensitivity and geometric limitations during application. Finally, like weld applied hardmetals, thermal spray coatings are similarly limited to solidification microstructures and subject to other process related microstructural constraints.
SOLID STATE HARDMETALS
The development of solid state densification powder metallurgy (SSDPM) processing of composite structures has enabled the fabrication of hardmetal inlays/overlays which potentially include a range of compositions and microstructures not attainable by solidification. In addition, SSDPM processing methodology also provides more precise control of macrostructural and microstructural features than that attainable with fused overlays, as well as lower defect levels. Such methods and resulting full coverage products are described in U.S. Pat. Nos. 4,365,679; 4,368,788; 4,372,404; 4,398,952; 4,455,278; and 4,593,776. However, the relatively slow hot isostatic pressing densification method entails onerous economic implications. It also is restricted to thermodynamically stable materials systems, effectively limiting the potential novelty attainable in composition and microstructure.
The advent of rapid solid state densification powder metallurgy (RSSDPM) processing of composite structures has enabled the fabrication of hardmetal inlays/overlays which include a much broader range of possible compositions and microstructures, as well as more favorable process economics. RSSDPM processing entails forging of powder preforms at suitable pressures and temperatures to achieve full density by plastic deformations in time frames typically of a few minutes or less. Such densification avoids the development of liquid phases and limits diffusional transport. For example, RSSDPM processing can be achieved by filling a flexible mold with various powders and other components to about 55% to 65% of theoretical maximum density, then compressing the filled mold in a cold isostatic press (CIP) at high pressure to create an 80% to 90% dense preform. This preform is then heated to about 2100 degrees F. and forged to near 100% density by direct compression using a particulate elastic pressure transmitting medium. Alternately, the final densification may be achieved by other rapid solid state densification processes, such as the pneumatic isostatic forging process described in U.S. Pat. No. 5,561,834.
Because the components are densified in stages, the size of the preform is significantly smaller than the interior of mold, and the finished part is significantly smaller than its corresponding preform, although each has about the same mass.
RSSDPM processing provides more precise control of microstructural features than that attainable with either fused overlays or slow-densified PM composites. Such fabrication methodologies for rock bits are disclosed in U.S. Pat. Nos. 4,554,130; 4,592,252; and 4,630,692. Shown in these patents and also in U.S. Pat. Nos. 4,562,892 and 4,597,456 are examples of drill bits with wear resistant hardmetal overlays which exploit the flexibility and control afforded by RSSDPM. None of these patents, however, teach or anticipate process-derived physical and microstructural specificity intrinsic to RSSDPM fabrication methods. Nor do they teach economic methods for fabrication or formulation strategies for optimization of full coverage RSSDPM inlays as a function of bit design and application.
Although many unique hardmetal formulations are made possible by RSSDPM, most will not be useful as rock bit hardmetal inlays because they lack the necessary balance of wear resistance, strength, and toughness. In addition, straight forward substitution of RSSDPM processing has been found to produce hardmetals which behave differently in service than their solidification counterparts. Some have exhibited unique failure progressions which disadvantage them for use in drilling service.
For example, a RSSDPM "clone" of a conventional weld applied hardmetal made from 65 wt. percent cemented carbide pellets (30/40 mesh WC-7% Co), and 35 wt % 4620 steel powder, was found to have lower crest wear resistance than expected due to selective hard phase pullout caused by shear localization cracking in the matrix. The presence of sharpened interfaces combined with the formation of ferrite "halos" around carbide pellets propitiates deformation instability under high strain conditions. Even though the primary characteristics normally used to evaluate hardmetal (volume fractions, pellet hardness, matrix hardness, and porosity) were superior to conventional material, the RSSDPM clone exhibited an unexpected weakness.
Other experimentation with RSSDPM hardmetal in drilling service has partially refuted conventional wisdom that maximization of volume fractions of hard phase increases robustness of cutting edges. In hard formations/severe service, tooth crests formulated with high carbide loading made possible with RSSDPM methods were found to be vulnerable to macro scale cracking. However, in locations where high velocity fluid erosion dominates such as water courses and jet-impinged cutter faces, carbide loading and particle size were pushed beyond conventional limits with increasing benefit.
In U.S. Pat. No. 5,653,299, a particular hardmetal matrix microstructure which is very advantageous for rolling cutter drill bits is shown. RSSDPM processing provides a cost effective, controllable way of achieving this matrix microstructure.
Optimization of RSSDPM hard metals entails consideration of both process derived and design derived specificities. The physical demands placed on hard metals differ with location on a bit, and are dependent on bit design characteristics as well as application conditions. In particular, the hardmetal formulations best suited to resist deformation, cracking, and wear modes operative at cutting edges or tooth crests are not optimal to resist abrasion, erosion, and bending conditions operating on cutter or tooth flanks. In turn, hardmetal formulations optimized for bit faces, watercourses, and gage faces will be similarly specific to local erosion, abrasion, wear, and deformation conditions.
POWDER METALLURGY FABRICATION METHODS
Forged, powder metal fabricated rock bits have been developed which incorporate composite powder preforms in the cold isostatic press (CIP) portion of the fabrication cycle in order to produce RSSDPM hardmetal inlays. U.S. Pat. No. 5,032,352, herein incorporated by reference, describes in detail a RSSDPM process particularly applicable to making components for earth boring bits. In particular, the patent describes the method of incorporating previously formed inserts in a mold prior to a CIP densification cycle to form a hardmetal inlay in the finished part. The inserts are usually molded using a powder binder mix in separate tooling.
One preferred method of making these mold inserts employs a metal injection mold process using sintered WC-Co cemented carbide particulate and steel powder bound with an aqueous polymeric fugitive binder such as methylcellulose. The resulting previously formed inserts are inserted into tooth recesses in the elastomeric CIP mold prior to filling with steel powder. After forging, the inserts become fully dense integral hardmetal inlays which can exhibit constituencies covering and exceeding ranges those attainable by various solidification means.
While forming a hard metal layer utilizing preformed insert structures offers performance potential not available via conventional processes, incorporation of preformed inserts requires close conformation to the flexible mold features, in order to provide dimensional control. This entails precision preform fabrication tooling and associated design effort. In addition, practical molding limits on section thickness, aspect ratios, and particle size and volume loading of carbide prevent very thin, very large, and very dense preformed inserts such as may be desirable to achieve the most cost effective and/or functional cutter overlay configurations.
In a completely different fabrication technology (infiltration), U.S. Pat. No. 4,884,477 describes the use of a fugitive adhesive on rigid female mold tooling for incorporation of hard material particulate species to achieve a superficial composite hard metal in PDC drag bit heads. This type of infiltration process typically uses a copper based binder material which melts at a temperature less than about 1000 degrees C. The melted binder fills the spaces between the powders packed within the mold and produces a part which has substantially the same dimensions as the interior of the mold. Also, copper based matrices exhibit lower yield strength and modulus of elasticity than those of the steel alloy matrices available in RSSDPM, making the infiltrated product inferior in service, particularly where significant strains are applied to the product in service. Also, in an infiltration process, the maximum practical attainable volume fraction of hard material particulate is limited to about 70 volume percent due to packing density limitations. Typically the volume percent actually attained is lower than 70%. This limits the wear and erosion resistance of the surface of the infiltrated product.
There is a need for a tough and very wear, abrasion and erosion resistant coating for the steel surfaces of drill bits. Preferably the coating will have a very high volume percent hard material particulate for good wear, abrasion and erosion resistance, and have a steel alloy matrix for strength and toughness. Ideally, the coating would be economical to form, even over large areas of the steel surfaces.